Table 1.

NKA activity in cell membranes isolated from cells pretreated with forskolin or OAG under low or high [Ca²⁺]_i conditions

	NKA activity (nmol mg−1 h−1)
	10 mM Na+	70 mM Na+
Low [Ca2+]i
Control	2285 ± 195	5211 ± 329
FSK + IBMX	1452 ± 217*	4228 ± 477*
OAG	1419 ± 162*	4044 ± 388*
High [Ca2+]i
Control	2132 ± 218	5043 ± 585
FSK + IBMX	2136 ± 376	5588 ± 877
OAG	2529 ± 492	5302 ± 954

COS cells stably expressing rat renal NKA were incubated with forskolin (FSK; 10⁻⁶ M) and IBMX (5 ± 10⁻⁴ M) or OAG (10⁻⁸ M) for 15 min at 37 °C at low (120 nM) or high (420 nM) [Ca²⁺]_i. Following incubation, membranes were isolated and permeabilized, and Na⁺, K⁺-ATPase activity was measured as ouabain-sensitive ATP hydrolysis in the presence of protein phosphatase inhibitors okadaic acid and FK506 at non-saturating (i.e. 10 mM) and saturating (i.e. 70 mM) levels of Na²⁺ as described in Methods. Values represent means ± S.E.M. of 7 separate experiments.

^*Significantly different from control, P < 0.01, Student's paired t test.